1. Field of the Invention
The present invention relates to an image forming method using a photothermographic material. More particularly, the invention relates to an image forming method using a photothermographic material forming a fusion image useful as a medical diagnosis image.
2. Description of the Related Art
In recent years, in the photographic image forming field, there has been a strong desire for providing a dry photographic development process from the viewpoints of protecting the environment and economy of space. In this field, from the standpoints of high sensitivity and high image quality, a silver halide photographic photosensitive material has conventionally been used. However, for the purpose of forming an image, after imagewise exposure, wet processing steps including, for example, a color development step, a desilvering bleaching processing step and a water washing stabilizing processing step, management of processing solutions for these steps and waste solution processing are required. These have been large obstacles for realizing convenient and rapid image forming.
Photothermographic materials utilizing organic silver salts are already known. Photothermographic materials have an image forming layer in which a reducible silver salt (for example, an organic silver salt), a photosensitive silver halide, and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder.
Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region.
On the other hand, as for color image forming methods, a method utilizing a coupling reaction between a coupler and an oxidization product of a developing agent is most common, and a photothermographic material adopting this method is described in U.S. Pat. Nos. 3,761,270 and 4,021,240, and Japanese Patent Application Laid-Open (JP-A) Nos. 59-231539 and 60-128438. In these patents, p-sulfonamide phenol has been used as a developing agent. All patents, patent publications, and non-patent literature cited in this specification are hereby expressly incorporated by reference herein. Since couplers do not have absorption in the visible region before processing, the photothermographic material based on a coupling method is more advantageous from the standpoint of sensitivity than a photothermographic material using a color forming material containing a dye that is already formed and is considered to be advantageous in that it can be used not only as a printing material but also as a photographing material. However, in the method of incorporating p-sulfonamide phenol, since p-sulfonamide phenol is deteriorated in the photothermographic material before development processing, there has been a problem in that an appropriate image can not be obtained. As for methods for solving this problem, photothermographic materials each containing a blocked p-phenylene diamine-type developing agent and processing methods therefor are proposed by European Patent (EP) Nos. 1,113,316A2, 1,113,322A2, 1,113,323A2, 1,113,324A2, 1,113,325A2, and 1,113,326A2; JP-A Nos. 2001-312026, 2003-215767 and 2003-215764; and U.S. Pat. No. 6,242,166.
However, in these photothermographic materials, since silver halide remains in a film after image forming, film turbidity and opacity become high due to light absorption and light scattering caused by such remaining silver halide, and therefore, such film is not appropriate for use in practically viewing a color image. For example, as shown in the Examples described in JP-A Nos. 2001-312026, 2003-215767 and 2003-215764; and U.S. Pat. No. 6,242,166, fogging becomes extremely high and is as high as 0.58 to 1.2. Further, in a portion in which the color image is formed, since a black image caused by developed silver is added on the thus-fogged image, it is difficult to directly view it as a color image. Therefore, as explained in the aforementioned references, the obtained image is a primary image and is not an image for being directly viewed, and accordingly, the image is digitalized, and image processing is performed to reduce fogging and adjust gradation and color tone, whereby it is attempted to form a reprocessed image which can be provided for viewing.
On the other hand, in the medical diagnostic image field, in recent years, the importance of diagnostic procedures for nuclear medicine such as PET (Positron Emission Tomography) and SPECT (Single Emission Computed Tomography) has been increasing. However, since there is scarce anatomic information available for these functional images, morphological images such as CT (Computed Tomography) and MRI (Magnetic Resonance Imaging) are separately photographed and placed on a display side by side, and a diagnosis is executed by comparing these images. Further, a technique for integrating the morphological image and the functional image on a display by utilizing software programming has been developed.
Since images are separately photographed in the above described technique, there are problems such as difficulty in registry of positioning and changes in images over time.
In order to solve these problems, in recent years, as described in Image Information Medical, November Issue, pp. 1302 to 1347 (2004), a PET-CT hybrid apparatus has been developed. Since the PET-CT hybrid apparatus can almost simultaneously photograph a PET image and a CT image and then place a fusion image made up of these images on a display, it is excellent in diagnostic performance.
Such fusion images are ordinarily represented on the display such that morphological images such as CT or MRI are shown in black-and-white while functional images such as PET or SPECT are shown in color. However, although these images can be displayed on a screen of a monitor, there is no device available at present which can produce a hard print with high precision and high quality such as can be obtained by a black-and-white laser printer for medical diagnosis. With the recent increasing social demand for open medical information and execution of an informed consent, preparation of a hard print of high quality has strongly been desired.